Add CancelRequest method to API

[dirkmc]

Currently the only way to cancel a graphsync request is by cancelling the context passed to Request().

It's not possible to determine when all events for the request have been processed. So the calling code may take actions that assume the request has completed when in fact some events are still being processed. This can cause races, for example filecoin-project/lotus#6968 (comment)

Proposal: Add a method to the graphsync API that cancels the request and waits for all event queues for the request to be drained:
CancelRequest(graphsync.RequestID)